Yol. 55.] GLACIAL PHENOMENA OF SPITSBERGEN. 683 



III. SuilFACE-PHENOMENA OP THE ICE-SHEETS. 



Although in some respects the surfaces of these ice-sheets are 

 not the best place to observe the action of ice, they are not without 

 their points of interest. In the upper portion near the radiating- 

 points the slope of the surface is gentle and snow-covered, becoming, 

 however, wavy and undulating as it approaches a nunatak ; in 

 places also eddies may be observed, distinctly comparable with the 

 swirl of river-water in the neighbourhood of submerged rocks, so 

 that, even where no rock is visible above the ice, the presence of 

 buried mountain-ridges may be often inferred. In immediate 

 proximity to a group of nunatakkr the ice is frequently much 

 crevassed, especially towards the lee side of the obstruction, the 

 fissures representing true marginal crevasses as the direct result of 

 differential ice-movement. Glacial lakes are also of frequent 

 occurrence here. On the impact-face ^ of a Jiunatak the ice is 

 much compressed and rises considerably above its average level 

 (PI. XLVI, fig. 2). This is undoubtedly due to the retardation of 

 the lower layers, and the increased shear-friction produced in the 

 upper ones : this surface- swelling therefore corresponds to the raised 

 lip which surrounds the snouts of many of the glaciers of the 

 country when advancing or when impeded by their old moraines, 

 as previously described.^ 



In connexion with this compression of the ice in the neighbour- 

 hood of the nunatakkr, an interesting phenomenon is sometimes 

 noticeable which does not appear to have been previously described. 

 When any crevassed portion of the ice-sheet is thrust against 

 such an obstruction, the pressure brought to bear causes the 

 crevasses to close, and an interesting peculiarity in the constitution 

 of the bridges that span the crevasses is forcibly brought to notice. 

 These bridges, instead of consisting of snow as do those which 

 occur in Swiss glaciers, are composed of solid ice, the result of 

 continuous melting in the summer and the action of severe 

 winter -frosts. When therefore the walls of the crevasse are 

 reunited, the ice-bridge which spans it is squeezed upward into the 

 form of an arch which finally becomes a complete tunnel : these 

 may be seen not infrequently running in parallel sets for long 

 distances (PI. XLYI, fig. 1) ; being broken in places, water readily 

 obtains access to them, and it is not an uncommon occurrence to 

 find these tunnels being made use of as drainage-channels for the 

 surface-water of the district. One of the most striking instances 

 observed by us occurred where the drainage from the Devonian 

 shales forming the flank of a nunatak coursed in a blood-red stream 

 between the blue translucent walls of a tunnel of this description 

 (PI. XLV). 



^ There being no recognized English equivalent in general use for the stoss- 

 seite of the German geologists, I have ventured to use for it the term impact- 

 face throughout this paper, since it commended itself to me as a fairly accurate 

 and expressive substitute. 



2 Quart. Journ. Geol. See. vol. liv (1898) p. 208 & pi. xviii. 



2t2 



